The present invention relates to a device for electrically correcting a shading or difference in infrared light quantity between the center and the periphery detector on an image sensor in the optical system of an infrared video camera.
FIGS. 5 and 6 show how a shading is produced in an infrared video camera. Designated at 1 is a lens, at 2 an infrared image sensor, such as an infrared charge coupled device (IRCCD), at 3 a cold shield for cutting off undesired infrared radiation, at 4 infrared rays falling upon the infrared image sensor 2, at 5 infrared rays passing through the lens 1, and at 6 infrared rays coming from the interior of an infrared video camera other than through the lens 1.
The cold shield 3 is cooled along with the infrared image sensor 2 to temperatures so low that the infrared emission from it is negligible. If the shading caused by the lens 1 is sufficiently small to neglect, a cause for producing a shading is a difference in intensity of the infrared rays 6 between the center and the periphery of the infrared image sensor 2. As is well known, the intensity is proportional to cos.sup.4 .theta. wherein .theta. is the incident angle of an infrared ray so that as is apparent from the figure, the intensity is higher in the center than in the periphery. Consequently, a positive shading effect is added to the output from the center of the infrared image sensor 2 regardless of the intensity of infrared rays 5 passing through the lens 1.
Where the lens 1 produces a considerably shading, there is a difference in intensity of the infrared rays 5 between the center and the periphery of the infrared image sensor 2. This difference is added to the above shading of the infrared rays 6. The magnitudes of the shading of the infrared rays 5 and 6 are determined by the quantity of infrared rays passing through the lens 1 from the outside and the temperature of the interior of an infrared video camera, respectively.
FIG. 7 shows a conventional device for correcting a shading in an infrared video camera. Denoted at 1 is a lens, at 7 an infrared detector, at 8 a shading pattern generator, and at 9 a subtractor. As described above in FIGS. 5 and 6, the output 55 of the infrared detector 7 is higher in the center than in the periphery of the image sensor because of the positive shading.
FIG. 8 shows a shading pattern generator 8a where the shading pattern produced in an infrared video camera is approximately by a two dimensional parabolic curve base on the fact that the shading pattern caused by a lens is generally approximated by a two dimensional parabolic curve which is symmetric about the optical axis. A parabolic waveform made in the vertical direction of a screen is multiplied by that of the horizontal direction to form a two dimensional parabolic waveform.
An integrating circuit 20 using an operational amplifier 22, to which a negative voltage (-V) is input, is reset every time a switch circuit 23 is switched in response to a synchronizing signal 31 in the vertical direction so that the output 32 of the integrating circuit 20 becomes a sawtooth waveform (a) as shown in FIG. 9(a). This sawtooth wave 32 is input to X and Y inputs of an analog multiplier 24 to provide at output 33 a parabolic waveform (b) in the vertical direction.
Similarly, in response to a synchronizing signal 34 in the horizontal direction, a switching circuit 28 of an integrating circuit 25, which includes an operational amplifier 27, switches to form a sawtooth wave (c) at output 35 in FIG. 9(b). This sawtooth 35 is input to an analog multiplier 29 to provide at output 36 a parabolic wave (d) in the horizontal direction. Finally, the vertical and horizontal parabolic waves (b) and (d) are input to an analog multiplier 30 to provide at output 37 a two dimensional parabolic wave (e) in FIG. 9(c).
FIG. 10 shows a shading pattern generator 8b where a shading caused in an infrared video camera takes a given shape. An address signal 42 represents coordinates in the vertical and horizontal directions. A shading pattern memory 40 has stored coordinates corresponding to shading quantities in the vertical and horizontal directions. In response to a coordinate address signal 42 corresponding to an output signal of the infrared detector 7, the shading pattern memory 40 outputs shading data 43. This shading data 43 is converted to a shading pattern analog data 44 by a D/A converter 41.
In the above infrared video camera shading correction device, the subtraction correction is carried out on the assumption of a fixed shading pattern. However, when the inside temperature of an infrared video camera changes under an influence of the varying outside temperature, etc., or when the average intensity of infrared rays from an object changes considerably, it is apparent from the above shading principle that the magnitude of shading also changes, bringing with it an error in the shading correction.